The documentation at http://www.mongodb.org/display/DOCS/CSharp+Driver+LINQ+Tutorial#CSharpDriverLINQTutorial-SupportedLINQqueryoperators doesn't seem to have been updated since 1.4. Here's some changes that I found in the release notes so far:

• .OfType<TModel>()
• .Any(x => x.Id == …) – included in above link, but also in 1.4.1 release notes
• DateTime comparisons
• scalar.In(arrayOrList) – Not a standard Linq extension method. You have to use a MongoDB namespace
• dictionary.ContainsKey(“key”)
• Nullable enums work
• Casting works

Here’s additional features that aren’t released yet but are in the works.
• .ToLowerInvariant() and .ToUpperInvariant() – 1.7 (completed already)
• “is” operator and typeof(TProperty) for properties – 1.7 (completed already)